Fluorinated sulfonyl fluorides having a fluoroformyl group are compounds useful as materials for ion-exchange resins. As a process for producing such compounds, there is a process of reacting a perfluoroalkylene oxide with a cyclic compound obtainable by a reaction of tetrafluoroethylene with sulfur trioxide (SO3) represented by the following scheme (the following scheme and Organofluorine Compounds, Chemistry and Applications, T. Hiyama et al., Springer-Verlag, Berlin, 2000, pages 228 to 230):

However, the conventional process is a disadvantageous process for practical industrial application, since due care is required for handling SO3. Further, the process is economically disadvantageous because the difficulty in synthesis is high. In addition, the reaction product is limited to a compound having a side chain (such as a —CF3 group), whereby there is a problem from the viewpoint of the performance and the membrane characteristics of an ion-exchange membrane.
As a process to solve the above problems, the following process is proposed (WO02/44138) wherein a hydrocarbon sulfonic acid derivative having a hydroxyl group is converted into an ester with a fluorinated carboxylic acid, which is directly fluorinated, and then pyrolyzed to obtain a fluorinated sulfonyl fluoride having a fluoroformyl group:

However, in this process also, since the starting material is limited to a compound derived from sulfonic acid such as isethionic acid, the skeleton of a compound to be obtained is limited.
Further, FSO2(CH2)2O(CH2)2OH to be used in the above process is known to be obtainable by a method of reacting NaOCH2CH2OH with FSO2(CH2)2Cl. Moreover, as another method, a method of fluorinating ClSO2(CH2)2O(CH2)2OH is also considered.
However, the former method has a problem in low yield of the product since NaOCH2CH2OH reacts also with the FSO2— group. Further, the latter method has such a problem that conditions in an oxidation step in the process for producing ClSO2(CH2)2O(CH2)2OH are disadvantageous for industrial application.